1. Field of Invention
This invention relates to improving print quality for fluid-jet printers.
2. Description of Related Art
In fluid jet printing, the fluid ejecting head typically includes one or more fluid ejectors. Each ejector includes a channel that communicates with a fluid supply chamber, or manifold, at one end of the fluid ejector, and with an opening at the opposite end of the fluid ejector. The opening at the opposite end of the fluid ejector is generally referred to as a nozzle. Fluid is expelled from each nozzle by known printing processes, such as “drop-on-demand” printing or continuous stream printing.
In a fluid jet printing apparatus, the fluid ejecting head typically includes one or more linear arrays of fluid ejectors. The fluid ejecting head is moved relative to the surface of the receiving medium, either by moving the receiving medium relative to a stationary fluid ejecting head, or vice versa, or both. In known fluid jet printing devices, a fluid ejecting head reciprocates across a receiving medium numerous times in the course of printing an image. Each pass of the fluid ejecting head across the receiving medium is referred to as a swath. There may be one or more fluid ejecting heads, one or more fluids ejected from each head, or some combination of both.
As the fluid ejecting head and the receiving medium are moved relative to each other, image-type digital data is used to selectively activate the fluid ejectors in the fluid ejecting head to generate a desired image.
A ubiquitous challenge in fluid-jet printing technology is the proper placement of fluid, such as, for example, ink on the receiving medium. The many manifestations of spot misplacement have many root causes. One example is the fluid ejecting head tilt error. The fluid ejecting head tilt error occurs when a line printed on the receiving medium that is intended to be perpendicular to the reciprocating motion of the head is slightly off-angle. In some cases, improper mechanical placement of the fluid ejecting head with respect to the carriage motion is responsible. Misalignment causes the defect of jagged edges in vertical lines and vertical edges, as well as other non-horizontal lines, which can span multiple swaths. While correct mechanical placement of the head is essential, there is also a more subtle cause dependent on the carriage speed, the fluid ejector firing sequence, and the time needed to step through one cycle of the firing sequence.
In modern printing devices or any other fluid jet printer which has hundreds or thousands of fluid ejectors, simultaneous operation of all nozzles requires prohibitively high currents and data rates, and would likely cause high fluidic cross-talk, degrading print quality. As a result, not all fluid ejectors are fired simultaneously. Instead, the fluid ejectors are partitioned into blocks, where each block consists of one or a small number of fluid ejectors. The fluid ejectors in a single block are fired simultaneously, and the blocks are fired sequentially. Then, for a fluid ejecting head moving left to right, the fluid fired from the fluid ejectors fired early on in a firing sequence will land on the receiving medium to the left of the fluid fired from fluid ejectors that are fired later in the firing sequence.
If the line of fluid ejectors is mechanically aligned perpendicular to the carriage direction, and if the firing sequence is such that the fluid ejectors located on the top of the fluid ejecting head are fired before the fluid ejectors situated below and printing occurs left to right, then the line of printed pixels will have a left-bent lean, as opposed to being arranged as a perfectly straight vertical line, as desired. Alternatively, when printing right to left, a vertical line tilted to the right may result. Alternatively, when printing right to left, with fluid ejectors at the bottom of the head firing before fluid ejectors situated at the top, a vertical line tilted to the left results. The magnitudes of the leans in all three examples are equal. When printing in both left to right and right to left directions, those skilled in the art will pair the left-bent leans rather than mixing left-bent and right-bent leans and introducing another defect.
A simple left-bent lean can be compensated for by design in the mechanical alignment of the head and carriage. Mechanical mechanisms, such as screws or micrometers, are known in the art to adjust the fluid ejecting head tilt. However, macroscopic mechanical tolerances are typically of the order of a 0.001 inches or 25.4 microns. This accuracy is not sufficient for the high image quality required by today's standards. The electronic system proposed in this patent has sub-micron resolution. Moreover, electronic solutions avoid mechanical hysteresis problems and are often inexpensive to implement.
Furthermore, the printer may be utilized in several print modes which require multiple carriage speeds and/or multiple times for one cycle of the firing sequence. Changing either the carriage speed or the time for one cycle of the firing sequence results in a changed tilt angle. A mechanical tilt in the design can typically compensate for only one tilt angle, not multiple tilt angles. The electronic system described in this patent permits a range of tilt angle settings for use at any time.
Other electronic means of tilt adjustment are known to those skilled in the art. For example, rotating the input image data allows one to effectively tilt the printhead by one pixel horizontally for each vertical printhead swath, or multiples thereof. The systems and methods of this invention allow a much more precise adjustment.
The problem of tilted lines occurs in all conventional print-on-demand printers, thermal ink jet printers, piezo ink jet printers, full width array printers, and any type of fluid ejecting head having multiple jets or nozzles.